An automobile such as a truck, a bus, or a construction machine controls a system such as the brakes or the suspension using compressed air, which is delivered from a compressor connected to the internal combustion engine. In some cases, an air drier is arranged downstream of the compressor to remove water and oil from the compressed air.
The air drier is filled with desiccant such as silica gel or zeolite. The air drier performs a loading mode operation, in which water is removed from the compressed air, and an unloading mode operation, in which the desiccant is regenerated. In the loading mode operation, the compressed air delivered from an inlet port of the air drier passes through the desiccant and is discharged from an outlet port of the air drier as dry compressed air. In the unloading mode operation, a drain valve device arranged in the air drier is opened to pass the dry compressed air through the desiccant in the direction opposite to that in the loading mode operation. This removes the oil and water that has been trapped by the desiccant and discharges the oil and water from the drain valve device as collected liquid.
The compressed air to be supplied to the compressor may be generated using a forced-induction device that is attached to an internal combustion engine or the like of a vehicle. This decreases the compression amount of air by the compressor, thus improving the efficiency and reducing the size of the compressor. However, if the inlet port of the air drier is open in the unloading mode operation, the compressed air that has flowed in from the inlet port is discharged from the drain valve device, which is open, into the atmospheric air. That is, the compressed air supplied from the forced-induction device is discharged into the atmospheric air through the compressor. The forced-induction pressure of the forced-induction device is thus decreased. To solve this problem, arranging a check valve (an inlet port opening/closing device) in the air drier has been proposed by which the inlet and outlet ports of the air drier are opened in the loading mode operation and closed in the unloading mode operation (see, for example, Patent Document 1).